1. Field of the Invention
The present invention relates generally to acoustic ink printing, and more particularly to an improved printhead having an acoustic reflection coating applied thereon to reduce unwanted transmission of acoustic energy into an ink pool.
2. Description of the Prior Art
Acoustic ink printing is a method for transferring ink directly to a recording medium having several advantages over other direct printing methodologies. One important advantage is the lack of necessity for nozzles and ejection orifices that have caused many of the reliability (e.g., clogging) and picture element (i.e., "pixel") placement accuracy problems which conventional drop-on-demand and continuous-stream ink jet printers have experienced.
As is known, an acoustic beam exerts a radiation pressure against objects upon which it impinges. Thus, when an acoustic beam impinges on a free surface (e.g., liquid/air interface) of a pool of liquid from beneath, the radiation pressure which it exerts will cause disturbances on the surface of the pool. The radiation pressure may reach a sufficiently high level that the force of surface tension is overcome and individual droplets of liquid are ejected from the pool. Given sufficient energy, the droplets may eject at a sufficient velocity to reach a recording medium proximately located to the free surface of the pool.
Focussing the acoustic beam on or near the surface of the pool intensifies the radiation pressure it exerts for a given amount of acoustic power. In order to accomplish such focussing, acoustic lenses are commonly used. These lenses conveniently are essentially spherically shaped indentations located in a substrate through which the acoustic beam may travel. One or more such lenses may be disposed in a single substrate, and each of the lenses may be individually addressable. See, for example, commonly assigned U.S. Pat. Nos. 4,751,529, and 4,751,534, both to Elrod et al. and issued June 14, 1988, and commonly assigned application for U.S. patent Ser. No. 07/253,371, filed Sept. 30, 1988, by Elrod et al., each incorporated by reference herein, for further discussion of acoustic lens characteristics.
Referring now to FIG. 1, there is illustrated (in pertinent part) an acoustic ink printhead 10 of a design known in the art. Acoustic ink printhead 10 includes a body or substrate 12. An acoustic wave generating means 14, typically a planar transducer, for generating an acoustic wave of predetermined wavelength is positioned on a lower surface 16 of substrate 12. Lower (and the like) is used herein for convenience and no limitation on orientation is intended thereby. Transducer 14 is typically composed of a piezoelectric film (not shown), such as a zinc oxide (ZnO), which is sandwiched between a pair of electrodes (also not shown), or other suitable transducer composition such that it is capable of generating plane waves 18 (explicitly shown in FIG. 1 for illustration) in response to a modulated rf voltage applied across its electrodes. Transducer 14 will typically be in mechanical communication with substrate 12 in order to facilitate efficient transmission of the generated acoustic waves into the substrate.
Acoustic lens 20 is formed in the upper surface 22 of substrate 12 for focussing acoustic waves 18 incident on its convex side to a point of focus 24 on its concave side. Upper surface 22 as well as the concave side of acoustic lens 20, face a liquid pool 26 (preferably an ink pool) which is acoustically coupled to substrate 12 and acoustic lens 20. This acoustic coupling may be accomplished by placing the liquid of liquid pool 26 in physical contact with acoustic lens 20 and upper surface 22, or by introducing between the liquid of liquid pool 26 and acoustic lens 20 and upper surface 22 an intermediate acoustic coupling media (not shown). Such intermediate acoustic coupling media are discussed in the aforementioned U.S. Pat. Nos. 4,751,534 ("Planarized Printheads For Acoustic Printing") and in copending Application for U.S. patent Ser. No. 07/287,791, filed Dec. 21, 1988, both commonly assigned.
When a printhead is formed having adjacent acoustic lenses, especially when the adjacent lenses are individually addressable, care must be taken to accurately direct the acoustic beam to impinge as exclusively as possible on the desired lens. Some of the undesirable effects of the acoustic beam impinging elsewhere than on the desired lens are insufficient radiation pressure on the liquid surface, lens cross-talk, and generation of unwanted liquid surface disturbances. Each of these effects result in loss or degradation of droplet ejection control. The present invention primarily addresses the later effect--generation of liquid surface disturbances.
As graphically shown in FIG. 1, plane waves 18 diverge as they radiate through the substrate from transducer 14 to upper surface 22. This divergence is due to the effect of diffraction of the sound wave passing through the substrate, and is a function of the radius of the transducer 14, of the thickness of the substrate, and of the wavelength of the wave passing through the medium. (It is generally assumed that the interface between substrate 12 and transducer 14 is ideal, so that consideration need not be given to the refractive effects of the wave passing from one medium to another, and further that transducer 14 generates a perfect plane wave.) The result of this divergence is to limit the center-to-center distance between adjacent lenses (if lenses are too closely spaced the diverging energy from one lens may impact an adjacent lens) and to cause energy to impinge upper surface 22 outside of lens 20 which may be imparted in the form of acoustic waves (not shown) into liquid pool 26.
Focus point 24, at or very near free surface 28, is the point of greatest concentrated energy for causing the release of droplet 30. Thus, by positioning the focus point 24 at free surface 28, the energy required to eject a droplet is minimized. However, focus point 24 is preset for each lens by the lens diameter, shape, etc. In order to maintain focus point 24 at or very near free surface 28, it is therefore important to maintain free surface 28 at a predetermined height above substrate 12.
As mentioned, one effect of illumination of surface 22 is transmission of radiant energy from substrate 12 to liquid pool 26. The radiant energy is transmitted through the liquid of liquid pool 26 striking free surface 28, thereby generating surface disturbances on free surface 28. These surface disturbances are transmitted along free surface 28 in the form of surface waves (not shown) which effect free surface 28 in regions directly above lens 20. In those cases where an array of lenses are used, the surface waves affect free surface 22 in regions above one or more acoustic lenses. The surface waves on free surface 28 result in deviation of free surface 28 from planar and from a preferred height, thereby altering the location of free surface 28 relative to fixed focus point 24, resulting in degradation of droplet ejection (i.e., print) control.
The result of free surface 28 deviating from planarity is varying angle of droplet ejection. Droplets will tend to eject in a direction normal to free surface 28. For optimum control of placement of the drop on the recording medium with the minimum amount of required acoustic energy, it is desired to maintain ejection angle of the drop at a predetermined valued, generally perpendicular to the local angle of the surface of the recording medium. Therefore, attempts are made to maintain free surface 28 parallel to the primary surface of the recording medium. Surface disturbances will vary the local surface angle of the liquid pool, especially over the acoustic lenses. This results is drop ejection at varying angles with consequent loss of printing accuracy and efficiency.
The result of free surface 28 varying from a preferred height is an increase in the energy required to cause droplet ejection and an adverse effect on droplet size and droplet ejection direction control. In fact, surface height must be maintained with a great deal of accuracy since acoustic waves entering liquid pool 26 will also reflect at free surface 28 resulting in coherent interference between the reflected and unreflected waves. The boundary conditions on free surface 28 for resonant constructive interference and anti-resonant destructive interference differ from each other by only one quarter wavelength. The effect of constructive interference is to exacerbate the surface disturbing effects of energy transmitted into liquid pool 26 outside lens 20.
Although it is possible that transducer size may be selected such that illumination outside lens 20 is minimized, changing transducer size impacts divergence of the wave in the substrate. For example, acoustic wave divergence effectively begins in a material after the distance d defined as EQU d=R.sup.2 /.lambda. (1)
where R is the radius of the transducer and EQU .lambda.=v.sub.m /f (2)
where v.sub.m is the velocity of sound in the material, and f is the frequency of the sound wave. If the transducer radius is decreased in order to reduce the size of the cone of divergence, the distance d from the transducer at which the divergence of the acoustic waves begins will be reduced. If the substrate thickness remains unchanged, decreasing transducer size (and hence reducing d) results in greater divergence. Thus, reducing the transducer size implies a reduction in substrate thickness. However, the thickness of the substrate is limited by its ability to support itself without cracking. This minimum thickness is on the order of 0.5-2 mm, and effectively limits the transducer size.
Similarly, it is possible to increase the radius of the acoustic lenses such that the diverging acoustic waves impinge fully on the lens. Typically, however, lens-to-lens spacing is much larger than the printed spot size. Thus, an array of lenses in staggered rows is often used for single pass printing. The result of increasing the center-to-center spacing is an increase in the number of staggered rows for a fixed print resolution. This is not desirable since it means that the printhead size (i.e., substrate size) and cost will both increase. Thus, this is also not an optimal solution.
Presently there is an unaddressed need in the art for improved performance of acoustic ink printing mechanisms. Specifically, there is a need in the art for a method and apparatus for minimizing surface disturbances at the free surface of the ink pool above one or more acoustic lenses. The invention described and contained herein addresses this and related needs in the art.